Comparing the impact of Mg²⁺ and Al³⁺ as dopants of LiNi_0.8Co_0.1Mn_0.1O₂ cathode for enhanced structural, interfacial, and thermal stability

Ni-rich layered oxide LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) is widely regarded as a promising cathode material for Li-ion batteries owing to its high energy density and relatively low cost, which are crucial for electric vehicle applications. However, its practical deployment is hampered by structural degradation and interfacial instability, primarily arising from Ni-induced cation disorder and oxygen release during cycling. While Mg²⁺ and Al³⁺ doping has individually been explored as effective strategies to mitigate these issues, a systematic comparison of their site-specific roles and the resulting impacts on structural, thermal, and electrochemical stability has remained limited. Here, we directly compare Mg²⁺- and Al³⁺-doped NCM811 at varied doping levels and elucidate their distinct site selectivity and mechanistic contributions. Rietveld refinement combined with slab-thickness analysis reveals contrasting substitutional behavior: Al³⁺ preferentially occupies the transition-metal (TM) sites, inducing TM-slab contraction and Li-layer expansion, whereas Mg²⁺ favors the Li sites, leading to Li-layer contraction (due to its smaller ionic size) and concomitant TM-slab expansion. At the optimized concentration of 1.0 mol%, both dopants enhance cycling stability, with Mg²⁺- and Al³⁺-doped samples achieving 96.5 % and 96.7 % capacity retention after 70 cycles at 0.1C, respectively, surpassing pristine NCM811 (95.3 %). Surface analysis further confirms that both dopants reduce residual lithium salts and suppress parasitic side reactions, thereby lowering charge-transfer resistance and improving rate capability. Specifically, the 1.0 mol% Mg-doped cathode exhibited significantly improved rate capability of 65.7 % capacity retention at 5C, compared to 50.8 % for pristine NCM811. Operando X-ray diffraction demonstrates that Mg²⁺ doping alleviates the abrupt interlayer contraction associated with the H2 → H3 phase transition at high states of charge. These findings highlight that site-specific lattice engineering via Mg²⁺ and Al³⁺ doping—when comparatively analyzed under identical conditions—offers complementary stabilization mechanisms. By integrating structural, thermal, and interfacial improvements, this study provides new insights into dopant-specific design principles for enhancing the durability of Ni-rich layered oxide cathodes.